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The development of passage systems for migratory fish is crucial to mitigate the impact of river fragmentation. Concerning downstream migration of juvenile salmon (smolts), understanding their behaviour is a key to improving the efficiency of bypass systems. Among devices to improve efficiency, artificial lighting has proved effective in certain situations. Based on (1) recent observations of early migrating smolts where migration was delayed in the Poutes dam reservoir (Allier River, France) and (2) the fact that the implementation of bypass lighting devices was based on experiments involving later-season migrants, the present study assessed the effect of a lighting device on wild early-migrating smolts. One hundred wild smolts were tagged with acoustic transmitters and their behaviour near the bypass entrance under lit or dark conditions was assessed using 2D acoustic telemetry. A very abrupt change in behaviour around mid-April was observed, which directly affected their response to light. In the first phase of the downstream migration season (before mid-April), lighting significantly reduced the attractiveness of the bypass, while this surprisingly seemed to favour passage: smolts less frequently approached the bypass entry zone but passed through it more frequently. However, in the second phase (after mid-April), lighting attracted and kept the smolts close to the bypass entrance and significantly increased passage, corroborating previous experiments. The present study demonstrated an interaction between the development of migratory fish and their behaviour under lit or dark conditions. It also highlighted the importance of taking account of such behavioural change during the migration season when designing fish passage systems.